A number of substances that are strong candicates for neurotransmitters in the mammalian CNS are found in high concentrations in the basal ganglia. These include acetylcholine (ACh), dopamine (DA), serotonin (5HT), gamma-aminobutyric acid (GABA), substance P and taurine. Little is known about the effect of these substances and other putative neurotransmitters on the responsiveness of neurons of the globus pallidus. In this investigation the responsiveness of primate pallidal neurons to microiontophoretically applied (7 barrel glass micropipettes) putative neurotransmitters was studied. Macaca mulatta monkeys anesthetized with with chloralose-urethane were used as the experimental animal. One hundred forty pallidal neurons have been studied to date; of these 101 were located in the internal segment, 29 in the external segment and 12 in the medullary lamina between the two segments. GABA, glycine, taurine and beta-alanine depressed the spontaneous firing of neurons located in all pallidal segments. GABA was the most powerful depressant and stopped spontaneous firing of many neurons with currents less than 5 nA. Cells located in the internal pallidal segment were generally more responsive to the depressant action of GABA than cells located in the external segment of laminar region. In general, glycine was not as effective a depressant as was GABa. L-glutamic acid was a weak excitant and failed to excite about 50% of the pallidal neurons to which it was applied. ACh was a weak to moderate excitant of most pallidal cells. DA was also a weak excitant and produced a delayed and prolonged excitation of many pallidal neurons. Noradrenaline, 5HT and substance P had little or no effect on the responsiveness of any pallidal neurons studied.